Mermaid built-in extension point to register further icons .

## Given

- an external iconset
- the vscode markdown mermaid built-in extension

## I Expect

- a way to register the external iconset via creating an extension

## Instead

- it seems I can't find an extension point for the built-in `mermaid` object

## Notes

I wanted to do something like that in an extension without overriding the whole markdown.*

```
...
mmd = ... extension point ...
mmd.registerIconPacks([
{ name: 'my:', icons: myIcons }
]);
...
```